Ytterbium-doped phosphate glass single mode photonic crystal fiber with all solid structure

1. Introduction

Rare-earth doped photonic crystal fiber (PCF) have been investigated for many years [1, 2]and one of the main subjects is to shorten the fiber length and enlarge the core diameter, which is especially important in single-frequency fiber laser [3] and ultrafast laser [4–6]. Many new fiber structures and techniques have been suggested to further enlarge the core diameter, including chirally-coupled-core (CCC) fibers [7], leaky channel fibers [8], gain-guiding fibers [9], photonic bandgap fiber [10], multi-trench fiber [11], large pitch fiber (LPF) [12], distributed modal filtering rod fiber [13], photonic crystal fibers (PCF) [14–17], and so forth. Up to now, a single-mode large mode area (LMA) LPF, a modified version of photonic crystal fiber, with a core diameter up to 135 µm [18] and a 40 cm LPF with output power 100 W have been demonstrated [12]. However, due to the presence of the hollows in a typical PCF [19–22], splicing the PCF with the fiber-coupled devices, such as pump source, is challenge. Therefore, the so-called all-solid photonic crystal fiber (AS-PCF) [23] was suggested to settle this issue and paves the way to build an all-fiber laser system. Furthermore, the rare-earth-solubility (RES) of soft glass is typically higher than in silica [24–28]and it is also remarkably convenient to adjust the index of the glasses [29–31], implying the potential of soft glass as the matrix of PCF. Consequently, one can expect a high absorption, short length, index-mismatching-free LMA-PCF by adopting the soft glass and the AS structure. After the suggestion of the first Nd³⁺-doped phosphate multimode AS-PCF [23], the Nd³⁺-doped silicate AS-PCF [32], Nd³⁺-doped phosphate single mode AS-PCF with core diameter up to 40µm [33], Yb/Er-doped AS-PCF [34], and Yb-doped AS-PCF [35]were realized.

In this paper, we demonstrate an Yb³⁺-doped phosphate single mode all-solid photonic crystal fiber with 8.2 W output power and 29% slope efficiency. The fiber has a core diameter of 15 µm and a length of 45 cm.

2. Experimental

Three kinds of phosphate glasses, Yb³⁺ doped glass with active dopant level of 6 wt.% (0.7 × 10²¹ ions/cm³) G0, and undoped glasses G1, and G2, were used to prepare the fiber. The composition of G0, G1, and G2, are P₂O₅-Al₂O₃-K₂O-BaO-Nb₂O₅-Sb₂O₃-La₂O₃-Yb₂O₃, P₂O₅-Al₂O₃-K₂O-BaO-Nb₂O₅-Sb₂O₃-La₂O₃-Y₂O₃, and P₂O₅-Al₂O₃-Na₂O-MgO-B₂O₃-La₂O₃-Y₂O₃, respectively. The refractive indices of G0, G1, and G2 are 1.5389, 1.5385, and 1.5120 respectively at 1.053 µm. The considerably large glass-forming region ensures a larger refractive index difference in the glasses, compared with silica, without the addition of any other dopants, such as germanium and fluorine [12, 27]. The absorption and emission cross section is shown in Fig. 1.The active glass has the largest absorption cross section of 1.16 × 10⁻²⁴ m² at 974 nm, and the emission cross section at 1046 nm, the emission wavelength of the fiber, is 1.2 × 10⁻²⁶. Please note that the emission wavelength red-shift from ~1000 nm in the bulk glass to 1046 nm in the fiber, which is also reported by Lee et al. [24]. Figure 2 shows the decay curve from which the lifetime was calculated to be ~984 µs.

 

Fig. 1 Absorption and emission cross-section spectra of the Yb³⁺-doped phosphate glass.

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Fig. 2 Decay curve of the Yb-doped phosphate glass.

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Stack-and-draw method was used to prepare the fiber preform [23].A G0 rod with diameter of 20 mm, aG1 tube with inner diameter of 14 mm and outer diameter of 20 mm, and two G2 rods with 14 mm and 20 mm in diameter, respectively, are fabricated. All of the rods and tube are made of the bulk glasses and have length of ~20 cm. Then, after appropriate acid treatment, the G1 tube and the G2 rod with 12 mm diameter are drawn into rods with 1 mm in diameter by the rod-in-tube method. The G0 rod and the G2 rod with 20 mm-diameter are also drawn into 1mm rods. After this, the rods were closed packed in a die and then the acquired fiber perform was fed into the fiber fabrication tower and drawn into fibers with desired outer diameters. Figure 3 shows the end-face of the fiber. The 1-cell core is composed of G0. The inner cladding consists of glasses G1 (light grey area) and G2 (dark grey dots). The outer cladding formed by glasses G2, which will contribute to resistant a higher heat load compared with the fiber coated with polymer. The ratio of the rod diameter, d, and the center-to-center distance between two nearest rods, Λ, is 0.7. A relatively large value of d/Λ and thereby a smaller core diameter of 15 µm was chose here because a larger core diameter, resulting from a small d/Λ value, necessitates a lower numerical aperture (NA) which induces a larger bend loss in our present 45-cm fiber. Further increasing the pump absorption, through raising the doping level and so forth, can contribute to shorten the fiber length and thus make the fiber with a larger core diameter immune to bend and other disturbance.

 

Fig. 3 The microscopic image of the AS-PCF.

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We used a fiber with 45 cm in length and an outer diameter of ~225 µm to build the laser. The NA and the normalized diameter of the inner cladding were 0.28 and ~120 µm, respectively. The doped core of the fiber as 15 µm in diameter, with calculated effective NA of ~0.055 at around 1 µm. Furthermore, the doped part has a loss of 2.3 dB/m at ~1330 nm and a pumping absorption coefficient of ~20 dB/m at 970 nm. Both properties were measured using the cutback method. A fiber-coupled laser diode operating at 970 nm was used as the pumping source. A collimating lens was applied to align the pumping beam, and a coupling lens with NA of 0.3 served as the laser input couple. The cavity was composed of a butt- coupled dichroic mirror with high reflectivity at around 1 µm, and a cleaved fiber end with ~4.5% Fresnel reflectivity.

3. Results and discussion

The laser performance and spectrum are shown in Figs. 4 and 5. A maximum output power of ~8.2 W is extracted with slope efficiency of 29%. No rollover is found at the highest output power, implying that the maximum power of our laser is only limited by the maximum available pump power. Please note that both the output power and slope efficiency were lower than the results reported by Lee et al. [24].To further figure out the performance of our fiber, an AS-PCF without doped core is used to measure the propagation loss of the pump power in the inner cladding to be as large as 10 dB/m, leading to an effective pump absorption of 10 dB/m for the doped core. Such high loss is ascribed to the impurity introduced when preparing the low-index rods in the PCC by rod-in-tube method and the fiber preform, and is the main reason reducing the output power as well as the slope efficiency. However, by improving the fabrication technology, such as processing the glass rods and the tube more sophisticatedly, the loss of the inner cladding can be reduced and thus the laser performance can be improved. The laser spectrum has a full width at half maximum (FWHM) of ~3 nm with the central wavelength at 1046 nm. The beam quality factor (M²) of the laser is measured to be 1.15. The M² factor and the far-field intensity profile of the laser are shown in Fig. 6..

 

Fig. 4 Measured laser output power plotted against the absorbed pump power.

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Fig. 5 Spectrum of the fiber laser.

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Fig. 6 Measured beam quality factors of the AS-PCF. Inset: beam in the far field.

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4. Conclusion

In conclusion, a phosphate single mode Yb-doped all-solid photonic crystal fiber with 15 µm was fabricated. 8.2 W output power with slope efficiency of 29% was obtained from a 45 cm long fiber. The low slope efficiency can be enhanced by decreasing the background loss through improving the fabrication technology.